diff --git a/msrp/responses.py b/msrp/responses.py
index be7befe..ed8b9d7 100644
--- a/msrp/responses.py
+++ b/msrp/responses.py
@@ -1,128 +1,140 @@
 
 from msrp.protocol import *
 
 def generate_report(code, forwarding_data, reason=None):
     from_data = forwarding_data.msrpdata_received
     report = MSRPData(generate_transaction_id(), method = "REPORT")
     report.add_header(ToPathHeader(from_data.headers["From-Path"].encoded))
     report.add_header(FromPathHeader([from_data.headers["To-Path"].decoded[0]]))
     report.add_header(StatusHeader((code, reason)))
     report.add_header(MessageIDHeader(from_data.headers["Message-ID"].encoded))
     start, end, total = forwarding_data.msrpdata_forward.headers["Byte-Range"].decoded
     if forwarding_data.bytes_received:
         end = start + forwarding_data.bytes_received - 1
         report.add_header(ByteRangeHeader([start, end, total]))
     return report
 
 def exception_from_data(data):
     try:
         response_exception = _response_exceptions[data.code]
     except KeyError:
         response = ResponseExceptionBase(data)
         response.code = data.code
         return response
     class ResponseExceptionWrapper(response_exception):
         def __init__(self, data):
             ResponseExceptionBase.__init__(self, data)
     return ResponseExceptionWrapper(data)
 
 class ResponseExceptionBase(MSRPError):
 
     def __init__(self, data):
         self.data = data
 
     def __str__(self):
         if self.data.comment:
             return "%03d %s" % (self.data.code, self.data.comment)
         else:
             return "%03d" % self.data.code
 
-class ResponseException(ResponseExceptionBase):
 
-    def __init__(self, code, request_data, comment = None, headers = []):
-        data = MSRPData(request_data.transaction_id, code = code, comment = comment)
-        for header in headers:
+class ResponseException(ResponseExceptionBase):
+    def __init__(self, code, request_data, comment=None, headers=None):
+        data = MSRPData(request_data.transaction_id, code=code, comment=comment)
+        for header in headers or ():
             data.headers[header.name] = header
         data.add_header(ToPathHeader([request_data.headers["From-Path"].decoded[0]]))
         data.add_header(FromPathHeader([request_data.headers["To-Path"].decoded[0]]))
         ResponseExceptionBase.__init__(self, data)
 
+
 class ResponseOK(ResponseException):
     code = 200
 
-    def __init__(self, request_data, headers = []):
+    def __init__(self, request_data, headers=None):
         ResponseException.__init__(self, self.code, request_data, "OK", headers)
 
+
 class ResponseUnintelligible(ResponseException):
     code = 400
 
-    def __init__(self, request_data, comment = None, headers = []):
+    def __init__(self, request_data, comment=None, headers=None):
         if comment:
             ResponseException.__init__(self, self.code, request_data, "Request was unintelligible, please try again (%s)" % comment, headers)
         else:
             ResponseException.__init__(self, self.code, request_data, "Request was unintelligible, please try again", headers)
 
+
 class ResponseUnauthenticated(ResponseException):
     code = 401
 
-    def __init__(self, request_data, headers = []):
+    def __init__(self, request_data, headers=None):
         ResponseException.__init__(self, self.code, request_data, "Unauthenticated", headers)
 
+
 class ResponseUnauthorized(ResponseException):
     code = 403
 
-    def __init__(self, request_data, comment = None, headers = []):
+    def __init__(self, request_data, comment = None, headers=None):
         if comment is None:
             ResponseException.__init__(self, self.code, request_data, "Unauthorized to use this relay", headers)
         else:
             ResponseException.__init__(self, self.code, request_data, "Unauthorized: %s" % comment, headers)
 
+
 class ResponseDownstreamTimeout(ResponseException):
     code = 408
 
-    def __init__(self, request_data, headers = []):
+    def __init__(self, request_data, headers=None):
         ResponseException.__init__(self, self.code, request_data, "Downstream transaction timed out", headers)
 
+
 class ResponseAbort(ResponseException):
     code = 413
 
-    def __init__(self, request_data, headers = []):
+    def __init__(self, request_data, headers=None):
         ResponseException.__init__(self, self.code, request_data, "Please abort the message you are sending", headers)
 
+
 class ResponseUnknownMediaType(ResponseException):
     code = 415
 
-    def __init__(self, request_data, content_type = None, headers = []):
+    def __init__(self, request_data, content_type=None, headers=None):
         ResponseException.__init__(self, self.code, request_data, "Unknown content type: %s" % content_type, headers)
 
+
 class ResponseOutOfBounds(ResponseException):
     code = 423
 
-    def __init__(self, request_data, parameter = None, headers = []):
+    def __init__(self, request_data, parameter=None, headers=None):
         if parameter:
             ResponseException.__init__(self, self.code, request_data, "Parameter out of bounds: %s" % parameter, headers)
         else:
             ResponseException.__init__(self, self.code, request_data, "Parameter out of bounds", headers)
 
+
 class ResponseNoSession(ResponseException):
     code = 481
 
-    def __init__(self, request_data, comment = None, headers = []):
+    def __init__(self, request_data, comment=None, headers=None):
         if comment is None:
             ResponseException.__init__(self, self.code, request_data, "Indicated session does not exist, please terminate", headers)
         else:
             ResponseException.__init__(self, self.code, request_data, "Indicated session does not exist, please terminate: %s" % comment, headers)
 
+
 class ResponseUnknownMethod(ResponseException):
     code = 501
 
-    def __init__(self, request_data, headers = []):
+    def __init__(self, request_data, headers=None):
         ResponseException.__init__(self, self.code, request_data, "Unknown method: %s" % request_data.method, headers)
 
+
 class ResponseSessionTaken(ResponseException):
     code = 506
 
-    def __init__(self, request_data, headers = []):
+    def __init__(self, request_data, headers=None):
         ResponseException.__init__(self, self.code, request_data, "This session is already bound to another network connection, stop sending messages", headers)
 
-_response_exceptions = dict((eval(cls_name).code,eval(cls_name)) for cls_name in globals() if cls_name.startswith("Response") and hasattr(eval(cls_name), "code"))
+
+_response_exceptions = dict((eval(cls_name).code, eval(cls_name)) for cls_name in globals() if cls_name.startswith("Response") and hasattr(eval(cls_name), "code"))